See http://www.openldap.org/software/release/changes.html
[2.3.31 fixes] Fixed slapd syncrepl logging (ITS#4755)
[2.3.30 fixes] Fixed slapd syncrepl consumer memory leaks (ITS#4746)
[2.3.29 fixes] Fixed libldap use keepalive for syncrepl (ITS#4708) Fixed slapd syncrepl modrdn new superior (ITS#4695) Fixed slapo-syncprov deadlock (ITS#4720)
Now, these are just the changes that say 'sync'. There are other likely relevant changes, such as a hdb livelock (#4738), that are fixed after 2.3.27. Quite a few of these changes are based off actual production impacting-bugs found at our various sites (new features are headed towards 2.4/HEAD, not 2.3). Can any of us say with certainty that you're going to run into these issues? Of course not. But if you _do_ run into those issues, it's going to feel pretty silly to have chosen to upgrade a few minor releases off from the already-known fix...